The present invention pertains to methods and apparatus for controlling the pressure within a supercritical processing system.
Supercritical fluids are effective mediums for reagent transport, reaction, and removal of impurities. Supercritical fluids are particularly useful for integrated circuit fabrication, including deposition of thin films and cleaning wafers and circuit devices. What is lacking in the art are more reliable and practical apparatus and methods of controlling pressure fluctuations in supercritical fluids during processing.
In conventional supercritical processing apparatus, pressure deviations from desired operating pressures are compensated for in different ways, depending on the deviation. For example, systems are typically vented for positive deviations in pressure. Negative deviations in pressure usually entail heating or pumping more fluid into the system to increase the pressure.
Each of the above-mentioned methods has drawbacks, for example, when trying to maintain a constant operating pressure of a supercritical solution containing a solvent and a solute (for example a chemical additive). Venting supercritical solutions causes loss of solute and solvent, potentially expensive materials. Pumping more solvent or solution into a system either dilutes the existing solution or at the very least requires additional material costs. Heating a supercritical solution to build pressure, for example after a pressure drop due to a solute addition, can take unnacceptable time periods or potentially destroy or decay heat sensitive solutes.
Additionally, in semiconductor processing for example, very tight pressure windows need to be maintained for uniform processes. For many solutes, there is a small pressure range in which the solute will remain soluble in a given supercritical solvent or solvent system. Adding to this issue, oftentimes addition of solutes causes a concomitant pressure change due to for example solubilization thermodynamics. When conventional apparatus fail to compensate for such pressure deviations quickly enough, solutes can precipitate or otherwise move out of phase with the supercritical medium.
What is needed therefore are improved methods and apparatus for controlling the pressure in a supercritical processing system. Preferably methods and apparatus that control pressure by changing the volume of a supercritical system during supercritical processing.
The present invention pertains to methods and apparatus for controlling the pressure in a supercritical processing system. Active methods for controlling the pressure include anticipating a pressure deviation due to a solute addition to a system, and changing the pressure within the system to compensate for the deviation. In this way, a desired pressure is achieved when the solute is added, without phase separation of the solute from the solvent. Pressure is adjusted by changing the volume of the supercritical processing system. Passive methods include adjusting the pressure of a supercritical system by changing the volume in response to a pressure deviation from a desired pressure. Apparatus for controlling the pressure in a supercritical processing system are described.
Thus, one aspect of the invention is an apparatus for controlling the pressure in a supercritical processing system. Such apparatus may be characterized by the following features: a pressure monitor configured to measure the pressure in the supercritical fluid processing system; a volume adjusting mechanism configured to variably change the volume of the supercritical processing system; and a controller configured to receive pressure information from the pressure monitor and deliver instructions to the volume adjusting mechanism, based on comparison of the pressure information with a pre-defined pressure.
Preferably the volume adjusting mechanism includes a vessel in fluid communication with the supercritical fluid processing system, the total volume of the supercritical processing system comprising the volume of the vessel; a piston contained within the vessel, the piston capable of a bidirectional movement within the vessel; and a driver for providing the bi-directional movement to the piston. Preferably the bidirectional movement of the piston causes either an increase or a decrease in the volume of the vessel, thereby causing a corresponding increase or decrease in the total volume of the supercritical processing system. Preferably the volume adjusting mechanism is a syringe pump. Preferably the vessel is in fluid communication with a process vessel, a recirculation loop, or a supercritical fluid supply line of the supercritical processing system. Preferably the response time for adjusting the pressure is between about 1 and 2 seconds, more preferably about 1 second.
Preferably controllers of the invention include a microprocessor or computer, each with an associated logic for receiving pressure information from the pressure monitor and delivering instructions to the volume adjusting mechanism, based on comparison of the pressure information with a pre-defined pressure. Also preferably, the total time required for a signal to pass from the pressure monitor, to the controller, and from the controller to the volume adjusting mechanism is between about 30 and 200 milliseconds, more preferably about 100 milliseconds. Preferably the controller is configured to instruct the volume adjusting mechanism to maintain the pre-defined pressure in the supercritical processing system, when a deviation of about xc2x1100 pounds per square inch from the predefined pressure is received by the controller, more preferably about xc2x125 pounds per square inch. Preferably the nominal operating pressure within the supercritical processing system is between about 1,000 and 5,000 pounds per square inch.
Another aspect of the invention is a method of controlling the pressure within a supercritical processing system. Such methods may be characterized by the following sequence: detecting a pressure deviation within the supercritical processing system from a pre-defined pressure to a second pressure; and changing the volume of the supercritical processing system to return the pressure within the supercritical processing system to approximately the pre-defined pressure. Preferably methods of the invention apply to nominal operating pressures within the supercritical processing system of between about 1,000 and 5,000 pounds per square inch. Preferably the pressure deviations within this range are about xc2x1100 pounds per square inch from the pre-defined pressure, more preferably about xc2x125 pounds per square inch.
Preferably changing the volume of the supercritical processing system to return the pressure within the supercritical processing system to approximately the predefined pressure is performed in between about 1 and 2 seconds more preferably about 1 second. Also preferably, in methods of the invention, changing the volume of the supercritical processing system to return the pressure within the supercritical processing system to approximately the pre-defined pressure is performed using a volume adjusting mechanism in fluid communication with the supercritical processing system.
Yet another aspect of the invention is a method of controlling the pressure within a supercritical processing system. Such methods may be characterized by the following sequence: receiving information for an anticipated pressure deviation, from a pre-defined pressure to a second pressure, due to a solute addition to a supercritical medium within said supercritical processing system; and applying a corrective pressure deviation, approximately equal in magnitude but opposite in value to said anticipated pressure deviation, to compensate for the anticipated pressure deviation. Preferably, the corrective pressure deviation is applied by changing the volume of the supercritical processing system.
Preferably, the nominal operating pressure within the supercritical processing system is between about 1,000 and 5,000 pounds per square inch. Also, preferably such methods further include choosing solute addition parameters so that the anticipated pressure deviation is about xc2x1100 pounds per square inch from the pre-defined pressure, more preferably about xc2x125 pounds per square inch from the pre-defined pressure.
Preferably active and passive methods of the invention are performed using apparatus of the invention as described herein. One particularly- preferred volume adjusting mechanism includes at least one syringe pump.
These and other features and advantages of the present invention will be described in more detail below with reference to the associated drawings.